In recent years, a ZnO-based semiconductor has been drawing attention as a material having higher multifunctionality than a nitride-based semiconductor containing nitrogen such as GaN, AlGaN, InGaN, InGaAlN, and GaPN.
The ZnO-based semiconductor is one of the wide-gap semiconductors, and has the following characteristics: having a markedly large exciton binding energy; being capable of existing stably at room temperature; being capable of emitting photons having excellent monochromaticity; and the like. Because of these characteristics, the ZnO semiconductor has been put into practical use as a material for: an ultraviolet LED used as a light source for lighting, backlighting, and the like; a high-speed electronic device; a surface-acoustic-wave device; and the like.
It is known, however, that a defect or the like may arise in such a ZnO-based semiconductor due to oxygen vacancies and interstitial zinc atoms. Such a crystal defect generates non-contributing electrons in the crystal, thereby causing the ZnO semiconductor to normally exhibit the n-type property. In order to obtain a p-type ZnO semiconductor, the concentration of the residual electrons needs to be decreased, and it is difficult to perform acceptor doping when a semiconductor device is formed by ZnO-based semiconductor layers. It is therefore difficult to form a p-type ZnO with good reproducibility.
However, it has been made possible to obtain a p-type ZnO that has good reproducibility, and light emission has been observed. Techniques of obtaining such a p-type ZnO are having been disclosed. For example, a p-type ZnO can be obtained as shown in Non-patent Document 1. In this technique, in order to obtain a semiconductor device using a ZnO semiconductor, a ScAlMgO4 (SCAM) substrate is used as a growth substrate, and −C surface ZnO is grown on a C surface of the SCAM substrate. The −C surface is also called an O (oxygen) polar surface. The wurtzite crystal structure that a ZnO crystal has no symmetry in the c-axis direction. The c-axis direction has two independent directions: +c and −c. The +c direction is also called a Zn polarity because Zn is located on the uppermost surface of the crystal in the +c direction; the −c direction is also called an O polarity because O is located on the uppermost surface of the crystal in the −c direction.
This −C surface ZnO is similarly grown on a sapphire substrate widely used as a ZnO crystal growth substrate. As shown in Non-patent Document 2 described by the inventors, in the crystal growth of a −C surface ZnO-based semiconductor, since the efficiency of doping nitrogen, which is a p-type dopant, depends largely on the growth temperature, the substrate temperature needs to be decreased to perform the nitrogen doping. However, decreasing the substrate temperature degrades the crystallinity, and then causes a carrier compensation center to be formed which compensates an acceptor nitrogen. Since nitrogen is therefore not activated, the formation itself of a p-type ZnO-based semiconductor layer is extremely difficult in the crystal growth of a −C surface ZnO-based semiconductor.
In this regard, as shown in Non-patent Document 1, there is a method which utilizes the dependency of the nitrogen doping efficiency on temperature. In the method, a p-type ZnO-based semiconductor layer having high carrier concentration is formed through temperature modulation in which the growth temperature is modulated between 400° C. and 1000° C. However, since the repetition of expansion and contraction due to the successive heating and cooling processes puts a heavy strain on a manufacturing apparatus, a large scale manufacturing apparatus is required. The method also causes a problem of requiring short maintenance cycle for the manufacturing apparatus. Moreover, a laser, which is not suitable for heating a large area, is used as a heating source. Accordingly, it is difficult to perform multiple-wafer growth to reduce device manufacturing cost.
As a measure for solving the above problems, we have already proposed a technique to form a p-type ZnO-based semiconductor having high carrier concentration by growing a +C surface ZnO-based semiconductor layer (refer to Patent Document 1). This patent publication is based on our finding that, in +C surface ZnO, the nitrogen doping has little dependency on the substrate temperature. In this technique, a +C surface GaN film to be a base layer is grown on the C surface of a sapphire substrate so as to be oriented in the +c-axis direction. Then, a +c-axis oriented ZnO-based semiconductor layer is formed on the +c-axis oriented GaN film, while inheriting the polarity of the +c-axis oriented GaN film. In this way, we have found out the non-dependency of the nitrogen doping on the growing temperature. Accordingly, the nitrogen doping can be carried out without reducing the substrate temperature, and as a result, the formation of the carrier compensation center can be prevented. A p-type ZnO-based semiconductor having high carrier concentration can thus be manufactured.    Patent Document 1: Japanese Patent Application Publication 2004-304166    Non-patent Document 1: Nature Materials vol. 4 (2005) p. 42    Non-patent Document 2: Journal of Crystal Growth 237-239 (2002) 503